(1) Field of the Invention
The present invention relates to pitot tubes for use in flow measurement systems. More particularly, the present invention relates to support and alignment of a traversing pitot tube.
(2) Description of the Prior Art
Pitot tubes are pressure sensing instruments used to determine the velocity of fluid flows. Depending on the arrangement, they can sample total pressure, static pressure, or both pressures simultaneously. Pitot tubes are widely used as airspeed indicators in the aviation industry and as gas flow meters in industrial applications.
A pitot tube consists of a tube oriented to direct fluid flow directly into, or across an orifice. Generally, pitot tubes are configured with a right angle bend, such that one leg is perpendicular to the flow and the other leg is extends into and parallel to the flow. One or more orifices are positioned within the leg that is parallel to the flow. The orifices are then connected to pressure sensing transducers or manometers.
If the orifice is directly aligned with the flow, the pitot tube samples stagnation, or total pressure. If the orifice is aligned perpendicular to the flow, the static pressure of the fluid is sampled. A single orifice of a pitot tube allows it to measure the fluid state at a single point, vice averaging over a large area.
An alternate version of the pitot tube, known as a pitot-static tube, samples both total pressure and static pressure simultaneously. The static signal can be subtracted from the stagnation signal to determine the dynamic pressure of the flow at that location. Then, for an incompressible flow with a known density, this dynamic pressure can be used to calculate velocity in a manner well known in the art.
A common technique used for determining the velocity profile within a square or round duct is known as a pitot traverse. The pitot traverse requires that a number of pitot-static measurements are taken at specific points perpendicular to the flow within the duct. Then, the magnitudes can be plotted to determine the velocity profile.
A velocity profile is typically used to calculate the overall average velocity and mass flow rate. It can be used to identify zones of recirculation or instability resulting from upstream obstructions, or to check for fully-developed flow.
However, performing a pitot traverse measurement across a large distance within a large diameter duct or under high velocity flow is problematic. As the pitot tube samples only at a single small location, the pitot tube must be cantilevered and extended into the flow further and further to reach sample points on the opposite side of the duct. These conditions can cause the orifice on the pitot tube to wander and vibrate during the measurement, whereas precise placement of the orifice within the duct is critical to conducting a proper pitot traverse.
Current designs rely on a single end support fitting mounted to the duct wall that attempts to hold the pitot tube in place. While they do help for the near-wall measurements, they are of little benefit for far-wall measurements, where the bending moment resulting from flow drag forces on the pitot tube is the greatest.
The use of an overly-stiff pitot tube to resist bending and flutter has been tried. However, for some large duct sizes and high fluid flow velocities, the size and stiffness required is impractical and typically unavailable. Velocity averaging pitot tubes have also been tried. These pitot tubes can be made significantly stiffer and extend all the way across the duct so as to be supported on both ends. However, velocity averaging pitot tubes sample at numerous locations and average readings into one pressure signal. They do not provide the flexibility or the fidelity of the single point sample of a standard pitot. Nor can they provide detailed velocity profile data.
Vibration, or singing, of the pitot tube is another problem that must be overcome. Vibration of a pitot tube results from fluid-dynamic forces generated by the oscillating vortex sheet behind the cantilevered tube. When the frequency of these oscillations matches the natural frequency of the pitot tube system and its support, resonance occurs. The effects of vibration for a cantilevered pitot tube have been found to result in significant over-indication of pressure with increases in vibration amplitude.
Again, previous solutions have relied on making the end support and tube prohibitively stiff. Common fluid-dynamic fixes such as trailing fins or strakes are also not feasible for a traversing pitot tube, as they would prohibit or severely limit translation.
Thus, a need has been recognized in the state of the art to provide a means to increase the accuracy of a pitot tube measurement probe when measuring the mass flow rate of a fluid within a duct using a traverse sampling technique. There is also a need to provide a means to minimize bending of the pitot tube when extended across the width of a duct. A further need exists to minimize vibration of the pitot tube during such extension of the tube.